The present invention relates to a method of controlling the air-fuel ratio of an internal combustion engine, and more particularly, to a method of this kind wherein the air-fuel mixture supplied to the engine is feedback-controlled to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics in approximate proportion to the exhaust gas ingredient concentration.
Among conventional methods of feedback-controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine (referred to hereinafter as "supply air-fuel ratio") to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics proportional to the exhaust gas ingredient concentration, there is a method proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 62-251443, in which a proportional term (P term), an integral term (I term), and a differential term (D term) are calculated based on a difference between an actual air-fuel ratio detected by the exhaust gas ingredient concentration sensor and a desired air-fuel ratio, and by the use of these calculated P, I, and D terms the supply air-fuel ratio is feedback-controlled.
However, according to this conventional method, feedback gains applied to calculation of the P, I, and D terms are set based on the engine rotational speed and the difference between the actual air-fuel ratio and the desired air-fuel ratio, but not set with other operating parameters of the engine taken into consideration. Therefore, the proposed method has the following disadvantage:
When the engine is in a predetermined decelerating condition, cutting-off of the fuel supply (hereinafter referred to as "fuel cut") is carried out. During fuel cut, the feedback control of the air-fuel ratio cannot be performed. Therefore, it is impossible to make the supply air-fuel ratio rapidly follow the desired air-fuel ratio immediately after termination of fuel cut, which may result in degradation of exhaust emission characteristics and driveability of the engine.